linux-kernel - Re: [PATCH 3/6] mm: free zapped tail pages when splitting isolated thp

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6622b7b4-e558-4d14-bc72-33e8008b06ec@gmail.com>
Date: Sun, 4 Aug 2024 20:02:50 +0100
From: Usama Arif <usamaarif642@...il.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: hannes@...xchg.org, riel@...riel.com, shakeel.butt@...ux.dev,
 roman.gushchin@...ux.dev, yuzhao@...gle.com, baohua@...nel.org,
 ryan.roberts@....com, rppt@...nel.org, willy@...radead.org,
 cerasuolodomenico@...il.com, corbet@....net, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, kernel-team@...a.com,
 Shuang Zhai <zhais@...gle.com>
Subject: Re: [PATCH 3/6] mm: free zapped tail pages when splitting isolated
 thp



On 30/07/2024 16:14, David Hildenbrand wrote:
> On 30.07.24 14:46, Usama Arif wrote:
>> From: Yu Zhao <yuzhao@...gle.com>
>>
>> If a tail page has only two references left, one inherited from the
>> isolation of its head and the other from lru_add_page_tail() which we
>> are about to drop, it means this tail page was concurrently zapped.
>> Then we can safely free it and save page reclaim or migration the
>> trouble of trying it.
>>
>> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
>> Tested-by: Shuang Zhai <zhais@...gle.com>
>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>> ---
>>   mm/huge_memory.c | 26 ++++++++++++++++++++++++++
>>   1 file changed, 26 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0167dc27e365..76a3b6a2b796 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2923,6 +2923,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>       unsigned int new_nr = 1 << new_order;
>>       int order = folio_order(folio);
>>       unsigned int nr = 1 << order;
>> +    LIST_HEAD(pages_to_free);
>> +    int nr_pages_to_free = 0;
>>         /* complete memcg works before add pages to LRU */
>>       split_page_memcg(head, order, new_order);
>> @@ -3007,6 +3009,24 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>           if (subpage == page)
>>               continue;
>>           folio_unlock(new_folio);
>> +        /*
>> +         * If a tail page has only two references left, one inherited
>> +         * from the isolation of its head and the other from
>> +         * lru_add_page_tail() which we are about to drop, it means this
>> +         * tail page was concurrently zapped. Then we can safely free it
>> +         * and save page reclaim or migration the trouble of trying it.
>> +         */
>> +        if (list && page_ref_freeze(subpage, 2)) {
>> +            VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
>> +            VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
>> +            VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
>> +
> 
> No VM_BUG_*, VM_WARN is good enough.
> 
>> +            ClearPageActive(subpage);
>> +            ClearPageUnevictable(subpage);
>> +            list_move(&subpage->lru, &pages_to_free);
> 
> Most checks here should operate on new_folio instead of subpage.
> 
> 
Do you mean instead of doing the PageLRU, PageCompound and page_mapped check on the subpage, there should be checks on new_folio?
If new_folio is a large folio, then it could be that only some of the subpages were zapped?

Could do below if subpage makes sense

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3305e6d0b90e..abfcd4b7cbba 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3041,9 +3041,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
                 * and save page reclaim or migration the trouble of trying it.
                 */
                if (list && page_ref_freeze(subpage, 2)) {
-                       VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
-                       VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
-                       VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
+                       VM_WARN_ON_ONCE_PAGE(PageLRU(subpage), subpage);
+                       VM_WARN_ON_ONCE_PAGE(PageCompound(subpage), subpage);
+                       VM_WARN_ON_ONCE_PAGE(page_mapped(subpage), subpage);
 
                        ClearPageActive(subpage);
                        ClearPageUnevictable(subpage);